Manifest constant delegates are mutable

[dlangBugzillaToGithub]

lngns reported this on 2018-11-04T23:53:26Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=19363

Description

The compiler does not check whether the function used to form a delegate mutates its context or not when declaring a manifest constant.

This should not work:
```
class C
{
	int member;
	int foo()
	{
		member++;
		return 123 + member;
	}
}

enum u = &(new C().foo);

void main()
{
    import std.stdio : writeln;
    writeln(u()); 	// 124
	writeln(u());   // 125
	writeln(u()); 	// 126
	writeln(u()); 	// 127
}
```

This was found by this user: https://forum.dlang.org/post/cfjftxwgnlaovaolbhop@forum.dlang.org

The expected behavior would be the statement `enum u = &(new C().foo);` failing for the expression `&(new C().foo)` is not constant, akin to assigning to const.

`const u = &(new C().foo);` => `Error: expression &C(0).foo is not a constant`

[dlangBugzillaToGithub]

contact commented on 2018-11-05T00:01:53Z

Actually, `const u = &(new C().foo);` fails even if C.foo does not mutate its object and is annotated const.
So I would suggest failing only for non-const methods, though that may be a bigger change.